1. Field of the Invention
The invention relates the field of electrochemical devices for storing electrical power.
In particular, it relates the batteries and the supercapacitors.
2. Description of Related Art
These devices are formed by the assembly of a plurality of electrochemical layers.
These devices generally include a multilayer unitary assembly that includes electrode layers (cathode and anode) and separation layers inserted between the electrodes. The device can also include one or more collector layers, each collector layer being in contact with certain electrode layers.
In the case of a liquid electrolyte device, the separation layer (separator) is formed from a porous material, and the different layers are impregnated with a liquid electrolyte solution.
In the case of a solid electrolyte device, the separation layer is composed of a solid electrolyte layer.
The function of the separation layer is to hold the electrode layers at a distance from each other so as to avoid a short-circuit of the electrochemical device while still allowing the establishment of an ion current between the electrode layers, through the electrolyte.
The function of each collector layer is to collect and conduct the current coming from an electrode that is associated with it.
In a storage device of the lithium-polymer battery type for example, the cathode layer(s) is (are) formed from a composition that includes a mixture of polymers and active charges. The polymer mixture typically contains a fluoridated polymer, such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). The active charges are composed of active charcoals or intercalation compounds of lithium for example.
The device includes one or more collector layer(s) associated with one or more cathode layer(s). The collector layer or layers are formed from metal, such as aluminium or copper.
In a supercapacitor type storage device, the electrode layer or layers are formed from a composition that includes a mixture of polymers and active charges. The polymer mixture typically contains a fluoridated polymer, such as polyvinylidene fluoride (PVDF) or the polytetrafluoroethylene (PTFE). The active charges are composed of active charcoals for example.
The device includes one or more collector layer(s) associated with one or more electrode layer(s). The collector layer or layers are formed from metal, such as aluminium or copper.
The metal collector layer or layers naturally have a tendency to former a superficial oxide film at their surface. Thus an aluminium collector layer forms a film of alumina (Al2O3) on the surface.
During the mechanical assembly of a collector layer on a cathode layer, the oxide film located at the interface between the collector layer and the cathode layer is broken.
During the operation of the electrochemical device, the ions of the electrolyte diffuse through the cathode up to the collector. The ions of the electrolyte react with the metal of the collector to form a passivation layer. Thus, the oxide film is progressively replaced or added to by the passivation layer obtained from reactions between ions contained in the electrolyte and the metal of the collector.
In the case of an aluminium collector, in the presence of fluoride (F) ions, the passivation layer typically contains aluminium fluoride (AlF3) or aluminium hydroxide (Al(OH)3).
In the presence of lithium ions (Li+), we observe the formation of a layer of lithium oxide (Li2O) or of lithium hydroxide (LiOH) as a replacement for the original oxide film.
Once formed, the passivation layer constitutes a barrier layer that prevents the diffusion of certain ions of the electrolyte up to the collector layer and thus protects the collector.
However, the formation of the passivation layer leads to consumption of the ions contained in the electrolyte and an attack on the collector layer.
In addition, in the case of a charged electrode, in the event of local rupture of the passivation layer (due to a shock or impact, a scratch or a chemical dissolution for example), the presence of graphitic charges within the electrode induces electrochemical reactions that dissolve the metal so as to bare the collector layer. These reactions can also produce potentially dangerous gas (dihydrogen in particular) by electrochemical battery-cell effect.
Finally, in the case of an aluminium collector layer and a polymer electrode layer, the presence of alumina at the surface of the collector layer leads to poor adhesion of the electrode layer onto the collector layer.
All of these phenomena can lead to complete consumption of the collector layer during the life of the power storage device and seriously damage the characteristics of the device in operation.
One aim of the invention is to preserve the collector layer during the lifetime of the electrochemical power storage device.